energy stored across the photosynthetic membrane (a
hydrogen-ion concentration gradient) to add inor-
ganic phosphate to ADP, thereby creating ATP. This
allows hydrogen ions (H+) to diffuse into the mito-
chondrion.
atrioventricular valve A valve in the heart between
each atrium and ventricle. It prevents a backflow of
blood when the ventricles contract.
atrium (plural, atria) An upper chamber that
receives blood from the veins returning to the verte-
brate heart and then pushes the blood to the ventricles,
the lower chambers. There is a left and right atrium.
Oxygenated blood returns from the lungs into the left
atrium and gets pushed down to the left ventricle. The
left ventricle pumps the blood out to the rest of the
body, transporting the oxygen to parts of the body that
need it. Blood returning from its voyage through the
body arrives in the right atrium. It then goes into the
right ventricle from which it goes through the lungs
again to get more oxygen, and the cycle continuously
repeats itself.
auranofin SeeGOLD DRUGS.
autacoid Abiological substance secreted by various
cells whose physiological activity is restricted to the
vicinity of its release; it is often referred to as local
HORMONE.
autogenesis model According to autogenesis (“self-
generating”), eukaryotic cells evolved by the specializa-
tion of internal membranes originally derived from
prokaryotic plasma membranes. This is another word
for spontaneous generation or abiogenesis.
autoimmune disease An immunological disorder in
which the immune system turns against itself. Autoim-
munity can be the cause of a broad spectrum of human
illnesses. Autoimmune diseases were not accepted into
the mainstream of medicine until the 1950s and 1960s.
They are diseases in which the progression from benign
autoimmunity to pathogenic autoimmunity happens
over a period of time and is determined by both genetic
influences and environmental triggers. Examples of
autoimmune diseases are idiopathic thrombocytopenic
purpura, Graves’ disease, myasthenia gravis, pemphi-
gus vulgaris (cause of pemphigus), and bullous pem-
phigoid (a blistering disease).autonomic nervous system(ANS) A division of the
nervous system of vertebrates. The nervous system con-
sists of two major subdivisions: the central nervous sys-
tem (CNS), made up of the brain and spinal cord, and
the peripheral nervous system (PNS), which comprises
ganglia and peripheral nerves outside the brain and the
spinal cord. The peripheral nervous system is divided
into two parts: the somatic, which is concerned with
sensory information about the environment outside the
body as well as muscle and limb position; and the auto-
nomic nervous system that regulates the internal envi-
ronment of vertebrates. It consists of the sympathetic
(fight/flight), parasympathetic (rest/rebuild), and enteric
nervous systems. The ANS is involved in the function
of virtually every organ system.
The parasympathetic nervous system takes care of
essential background operations such as heart/lungs
and digestion, while the sympathetic nervous system
provides stress-response and procreation strategies and
functions. The enteric nervous system takes care of
controlling the function of the gut.
The sympathetic nerves form part of the nerve net-
work connecting the organ systems with the central ner-
vous system. The sympathetic nerves permit an animal
to respond to stressful situations and helps control the
reaction of the body to stress. Examples of the sympa-
thetic reactions are increase in heart rate, decrease in
secretion of salivary and digestive glands, and dilation
of pupils. The parasympathetic nerves connect both
somatic and visceral organs to the central nervous sys-
tem, and their primary action is to keep body functions
normalized. The ANS works to conserve the body’s
resources and to restore equilibrium to the resting state.autophytic The process whereby an organism uses
photosynthesis to make complex foods from inorganic
substances.autophytic 31